[llvm-commits] [llvm] r147391 - in /llvm/trunk: lib/Transforms/InstCombine/InstCombineShifts.cpp test/Transforms/InstCombine/shift.ll

[Nick Lewycky]

On 01/01/2012 06:24 AM, Duncan Sands wrote:
> Hi Nick,
>
>> @@ -603,9 +602,16 @@
>>          // (X>>? C1)<<   C2 -->   X>>? (C1-C2)&   (-1<<   C2)
>>          if (I.getOpcode() == Instruction::Shl&&
>>              ShiftOp->getOpcode() != Instruction::Shl) {
>> -        Value *Shift = Builder->CreateBinOp(ShiftOp->getOpcode(), X,
>> -                                            ConstantInt::get(Ty, ShiftDiff));
>> -
>> +        ConstantInt *ShiftDiffCst = ConstantInt::get(Ty, ShiftDiff);
>> +        if (ShiftOp->isExact()) {
>> +          // (X>>?exact C1)<<   C2 -->   X>>?exact (C1-C2)
>
> what happens if C2 is bigger than C1?

We won't reach here. It extracts C1 and C2 into ShiftAmt1 and ShiftAmt2 
and does if (ShiftAmt1 == ShiftAmt2) { ... } else if (ShiftAmt1 < 
ShiftAmt2) { ... } else { our transform here }.

Nick

>
> Ciao, Duncan.
>
>> +          BinaryOperator *NewShr = BinaryOperator::Create(ShiftOp->getOpcode(),
>> +                                                          X, ShiftDiffCst);
>> +          NewShr->setIsExact(true);
>> +          return NewShr;
>> +        }
>> +        Value *Shift = Builder->CreateBinOp(ShiftOp->getOpcode(),
>> +                                            X, ShiftDiffCst);
>>            APInt Mask(APInt::getHighBitsSet(TypeBits, TypeBits - ShiftAmt2));
>>            return BinaryOperator::CreateAnd(Shift,
>>                                             ConstantInt::get(I.getContext(),Mask));
>>
>> Modified: llvm/trunk/test/Transforms/InstCombine/shift.ll
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/shift.ll?rev=147391&r1=147390&r2=147391&view=diff
>> ==============================================================================
>> --- llvm/trunk/test/Transforms/InstCombine/shift.ll (original)
>> +++ llvm/trunk/test/Transforms/InstCombine/shift.ll Sat Dec 31 15:30:22 2011
>> @@ -542,3 +542,21 @@
>>    ; CHECK-NEXT: %y = lshr i32 %a, 5
>>    ; CHECK-NEXT: ret i32 %y
>>    }
>> +
>> +define i32 @test46(i32 %a) {
>> +  %y = ashr exact i32 %a, 3
>> +  %z = shl i32 %y, 1
>> +  ret i32 %z
>> +; CHECK: @test46
>> +; CHECK-NEXT: %z = ashr exact i32 %a, 2
>> +; CHECK-NEXT: ret i32 %z
>> +}
>> +
>> +define i32 @test47(i32 %a) {
>> +  %y = lshr exact i32 %a, 3
>> +  %z = shl i32 %y, 1
>> +  ret i32 %z
>> +; CHECK: @test47
>> +; CHECK-NEXT: %z = lshr exact i32 %a, 2
>> +; CHECK-NEXT: ret i32 %z
>> +}
>>
>>
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